It has been surprisingly been found that the compounds of general formula I are positive allosteric modulators (PAMs) of metabotropic glutamate receptor 4 (mGluR4).
Metabotropic glutamate receptor 4 is a protein that in humans is encoded by the GRM4 gene. Together with GRM6, GRM7 and GRM8 it belongs to group III of the Metabotropic glutamate receptor family, and is negatively coupled to adenylate cyclase via activation of the Gαi/o protein. It is expressed primarily on presynaptic terminals, functioning as an autoreceptor or heteroreceptor and its activation leads to decreases in transmitter release from presynaptic terminals. mGluR4 is currently receiving much attention based primarily upon its unique distribution and the recent evidence that activation of this receptor plays key modulatory role in many CNS and non-CNS pathways (Celanire S, Campo B, Expert Opinion in Drug Discovery, 2012)
The similarity in the ligand binding domains of group III mGluRs creates a challenge for identifying selective orthosteric agonists of this receptor, although some progress has been made in this area. However, targeting positive allosteric modulators (PAMs) rather than orthosteric agonists provides a broader opportunity to identify molecules that are exclusively selective between mGluRs.
mGluR4 PAMs are emerging as promising therapeutic agents for the treatment of motor (and non motor) symptoms as well as a disease-modifying agent in Parkinson's disease through a non-dopaminergic approach.
Parkinson's disease is a progressive neurodegenerative disease that results in the loss of dopaminergic neurons in the substantia nigra (SN). One consequence of the depletion of dopamine in this disease is a series of movement disorders, including bradykinesia, akinesia, tremor, gait disorders and problems with balance. These motor disturbances form the hallmark of PD, although there are many other non-motor symptoms that are associated with the disease. Early in the course of the disease, PD symptoms are effectively treated by dopamine replacement or augmentation, with the use of dopamine D2 receptor agonists, levodopa or monoamine oxidase B inhibitors. However, as the disease progresses these agents become less effective in controlling motor symptoms. Additionally, their use is limited by the emergence of adverse effects including dopamine agonist-induced dyskinesias. Consequently, there remains a need for new approaches to the treatment of PD that improve the effectiveness of the control of motor symptoms.
Activation of metabotropic glutamate receptor 4 (mGluR4) has been proposed as a potential therapeutic approach to Parkinson's disease. A member of the group III mGluRs, mGluR4 is predominantly a presynaptic glutamate receptor that is expressed in several key locations in the basal ganglia circuits that control movement. Activation of mGluR4 with group III-preferring agonists decreases inhibitory and excitatory post synaptic potentials, presumably by decreasing the release of GABA and glutamate respectively.
The search for novel drugs that relieve motor symptoms of Parkinsonism whilst attenuating the ongoing degeneration of nigrostriatal neurons is of particular interest. Orthosteric mGluR4 agonist L-AP4 has demonstrated neuroprotective effects in a 6-OHDA rodent model of PD and first positive allosteric modulator (−)-PHCCC reduced nigrostriatal degeneration in mice treated with 1-Methyl-4-Phenyl-1,2,3,6-Tetrahydropyridine (MPTP). These studies provide convincing preclinical evidence suggesting that mGluR4 activators constitute a valid approach not only for symptomatic treatments of PD, but also potentially as disease modifiers for this indication.
The neuroprotective effects of selective mGluR4 modulators was also described in Neuroreport, 19(4), 475-8, 2008, Proc.Natl. Acad. Sci, USA, 100(23), 13668-73, 2003 and J.Neurosci. 26(27), 7222-9, 2006 and Mol. Pharmacol. 74(5), 1345-58, 2008.
Anxiety disorders are among the most prevalent psychiatric disorders in the world, and are co-morbid with Parkinson's disease (Prediger R, et al. Neuropharmacology 2012;62: 115-24). Excessive glutamatergic neurotransmission is one important feature of anxiety pathophysiology. Based on presynaptic localization of mGluR4 in brain areas involved in anxiety and mood disorders, and dampening excessive brain excitability, the mGluR4 activators may represent a new generation of anxiolytic therapeutics (Eur. J. Pharmacol., 498(1-3), 153-6, 2004).
Addex has reported in 2010 that ADX88178 was active in two preclinical rodent models of anxiety: the marble burying test in mice and EPM in mice and rats. ADX88178 also displayed an anxiolytic-like profile in the rat EPM test after oral dosing.
mGluR4 modulators were also shown to exert anti-depressive actions (Neuropharmacology, 46(2), 151-9, 2004).
In addition, mGluR4 modulators were also shown to be involved in glucagon secretion inhibition (Diabetes, 53(4), 998-1006, 2004). Therefore, orthosteric or positive allosteric modulators of mGluR4 have potential for the treatment of type 2 diabetes through its hypoglycemic effect.
Moreover, mGluR4 was shown to be expressed in prostate cancer cell-line (Anticancer Res. 29(1), 371-7, 2009) or colorectal carcinoma (Cli. Cancer Research, 11(9)3288-95, 2005). mGluR4 modulators may therefore have also potential role for the treatment of cancers.
Other proposed effects of mGluR4 PAM's can be expected for the treatment of emesis, obsessive compulsive disorder, anorexia and autism.